got nil pointer when using costom JSONSchema func

[eternal-dragon]

func (Coustom) JSONSchema() *jsonschema.Schema { return &jsonschema.Schema{ Type:"string", } }

I just add this func to my coustom struct, and got a nil pointer from here:

github.com/invopop/jsonschema.(*Reflector).reflectCustomSchema(0x7f6855c136f8?, 0x7f687f7d8108?, {0xbe1478, 0xad3980})
/home/XXX/go/pkg/mod/github.com/invopop/[email protected]/reflect.go:423 +0xb0
github.com/invopop/jsonschema.(*Reflector).reflectTypeToSchema(0xc00021fa58, 0xc00021f960?, {0xbe1478, 0xad3980})
/home/XXX/go/pkg/mod/github.com/invopop/[email protected]/reflect.go:348 +0x89
github.com/invopop/jsonschema.(*Reflector).reflectTypeToSchemaWithID(0xc00021fa58, 0xbe1478?, {0xbe1478, 0xad3980})
/home/XXX/go/pkg/mod/github.com/invopop/[email protected]/reflect.go:324 +0x2c
github.com/invopop/jsonschema.(*Reflector).ReflectFromType(0xc00021fa58, {0xbe1478, 0xad3980})
/home/XXX/go/pkg/mod/github.com/invopop/[email protected]/reflect.go:244 +0xf3
github.com/invopop/jsonschema.(*Reflector).Reflect(0xb0?, {0xad3980?, 0xc0000c3340?})
/home/XXX/go/pkg/mod/github.com/invopop/[email protected]/reflect.go:230 +0x4a

I tried func (*Coustom) JSONSchema() *jsonschema.Schema ,but still got the same error.

Did I make a mistake somewhere?

[eternal-dragon]

And if I try to add type on field, it will be like :
"XXX": { "$ref": "#/$defs/Coustom", "type": "string" },
It has both type and ref. I want it only have type.